Hi, I'm not a game dev and I'll only be playing the games resulting from this jam... because I know nothing about development and nothing about coding, but I'm interest in your idea especially as an ace myself. Or, well, ace-spec.
I have a few questions that might help you with the characterization: is your protagonist romantic or aromantic? Are they on the "completely asexual" end of the spectrum or do they experience sexual attraction sometimes, or given certain conditions (like with demisexual people they only experience sexual attraction once a strong emotional bond is established)? Since you said every character is going to be ace, I'd personally like it if each one experienced asexuality differently (and there are a lot of variables to pick from: you have aromantic asexuals, heteroromantic asexuals, homoromantic asexuals, gray-aces, demisexuals, aromantic asexuals... also asexuals with high libido or non-existent libido, asexuals who are sex-repulsed or not, asexuals who just don't care, and these are just variables off the top of my head).
So, if their orientation is a relevant theme in the game, I suggest making yourself questions about how these characters feel about attraction, and since they're all asexuals I imagine their web of relationships would be very different from the kind of relationships we're generally used to (like, more on the platonic side?).
Also, you probably know this but I think it's very important to say that ace people can choose to have sex with someone for reasons outside of attraction, like having a child or making a bond with someone get stronger (given the multiple purposes sex can have, there are just as many reasons why someone would want to have it)...
But other than this, I'd say write the characters like you would any other. In the end, sexuality is only a marginal part of someone's identity and unless you're going to center the narration around it you don't need to worry about it too much.
I really like that the choices will be more about agency than "making the right one", btw! This is gonna be interesting. Hope I didn't say anything superfluous and that I was of help, in any way!